Switching elements, such as IGBTs, MOSFETs, and so on, are commonly used for turn-on and turn-off of devices, power conversion, and the like. Such switching elements are sensitive in their reliability to their temperatures, so drive of these switching element should be preferably controlled based on their temperatures.
In view of these circumstances, Japanese Patent Application Publication No. 2009-171312 is known, which discloses a system in which IGBTs as an example of switching elements are provided to convert inputted DC power into AC power, and the AC power is supplied to a high-power load, such as a motor-generator. Turn-on or turn-off of each of the IGBTs is controlled by a microcomputer installed in the known system. In the known system, because of the high-power load, a high-voltage system including the high-power load and the IGBTs and a low-voltage system including the microcomputer are separated from each other, and the high- and low-voltage systems are communicably coupled with each other via communication media. Particularly, in the known system, photocouplers are used as the communication media, so that electrical insulation is established between the high- and low-voltage systems while communications can be maintained therebetween.
The known system is designed such that information about temperature of each IGBT of the high-voltage system is transferred to the microcomputer of the low-voltage system via a corresponding photocoupler.
Specifically, the first voltage system is equipped with a temperature-sensitive diode and a frequency modulator for each IGBT. The temperature-sensitive diode measures a temperature value of a corresponding IGBT, and sends, to the frequency modulator, an output signal indicative of the measured temperature value of the corresponding IGBT. The frequency modulator frequency-modulates the output signal from the temperature-sensitive diode, and sends the frequency-modulated signal to a corresponding photocoupler. The photocoupler converts the frequency-modulated signal into light, returns the light to an electric output signal corresponding to the frequency-modulated signal, and sends the output signal to the microcomputer. The microcomputer receives the output signal sent from the photocoupler, and calculates a temperature value of the corresponding IGBT based on the received output signal.
If it determines that the calculated temperature value of an IGBT exceeds a preset threshold value, the microcomputer determines that the IGBT is in a specified temperature state, so that drive of the IGBT should be limited. Then, the microcomputer performs a task to limit drive of the IGBT. This task limits a collector current from flowing as an output current through the IGBT, thus avoiding the reduction in the reliability of the IGBT due to its overheating.